1. Field of the Invention
The present invention relates to a radio transmitting apparatus which transmits signals to an adaptive array antenna, and a gain control method for the radio transmitting apparatus.
2. Description of the Related Art
An adaptive array antenna transmitting apparatus, which is well known as a radio transmitting apparatus, carries out transmission with directivity by transmitting the same signal from a plurality of antennae while changing the amplitude and phase of the signal. The process for altering the amplitude and phase can be accomplished by perform multiplication on an analog signal or by perform multiplication on a digital signal. Because the process on a digital signal has a higher precision than on an analog signal, the multiplication is often executed on a digital signal by using a complex multiplier.
FIG. 5 exemplifies an adaptive array antenna transmitting apparatus. As illustrated, this apparatus performs modulation on a transmission signal S by means of a baseband modulator 501, and then performs vector multiplication with different complex weight coefficients W.sub.1, and W.sub.2 by means of vector multipliers 502 and 503. The signals resulting from the multiplication are converted to analog signals by D/A (Digital-to-Analog) converters 504 to 507. The analog signals are subjected to orthogonal modulation by orthogonal modulators 508 and 509, and then filtered by band-pass filters 510 to 513. The filtered signals are amplified by power amplifiers 514 and 515 and are then transmitted from antennae A and B.
The orthogonal modulators 508 and 509 used in the above process have a modulation characteristic as shown in FIG. 6 with respect to the input signal level. The characteristic is such that the modulation precision becomes equal to or greater than .beta., which is a practical range, when the input signal level lies between (.alpha.-.DELTA..sub.1) and (.alpha.+.DELTA..sub.2), and the modulation precision becomes the highest when the input signal level is .alpha..
The adaptive array antenna transmitting apparatus transmits a signal multiplied by a complex weight coefficient W.sub.1 antenna by antenna. When the amplitude .vertline.W.sub.m .vertline. of the complex weight coefficient is small, therefore, inputs to the orthogonal modulators become smaller, whereas when the amplitude .vertline.W.sub.m .vertline. of the complex weight coefficient is large, inputs to the orthogonal modulators become large. When the amplitude .vertline.W.sub.m .vertline. of the complex weight coefficient is too small or too large, therefore, inputs to the orthogonal modulators do not fall in the range from (.alpha.-.DELTA..sub.1)to (.alpha.+.DELTA..sub.2), thereby reducing the modulation precision of the transmitting apparatus.
Accordingly, it is an object of the present invention to provide an adaptive array antenna transmitting apparatus with a high modulation precision.